1. Field of the Invention
The present invention relates to an improved golf club, to an improved method for manufacturing a golf club, and more specifically to conditioning the impact face of a golf club with deep compressive stress using laser peening.
2. Description of Related Art
Shot peening has been suggested for club faces to provide better control and increased distance for golf shots. See, U.S. Pat. No. 5,487,543 by Funk. In particular, Funk describes shot peening the face of a club to impart residual compressive stress and harden the surface of the club, and claims that doing so improves the feel of the club by reducing vibration. Funk suggests that shot peening reduces the coefficient of friction of, and hardens, the club face. Funk applied relatively shallow compressive stress by shot peening on the front surface of the club face, with a peak amount of about 165 MPa at a depth of about 0.002 inches (0.043 mm). We note here that an MPa is a million Pascals. 165 MPa converts to about 24,000 pounds per square inch.
U.S. Pat. No. 6,994,635 by Poyner also teaches shot peening golf club heads “to increase hardness and residual compressive stress.” Poyner, column 3 lines 24-25. However, the Poyner patent focuses on shot peening for the back surface of the ball striking surface to remove [alpha]-case, and to improve fatigue limits. In Poyner, it is suggested that the residual compressive stress in a back surface of a club face should be “as great as about sixty percent of the yield stress of a typical golf ball, which is about 62 MPa.” Poyner, column 4, lines 64-67. Poyner then states, “Residual stresses exceeding 500 MPa may be produced by shot peening of titanium.” Poyner, column 5, lines 5-6. This comment by Poyner appears to be a statement intended to show that shot peening is readily capable of inducing compressive stresses on the back surface of a club face in the range of “as great as about sixty percent of the yield stress of a typical golf ball . . . ”, rather than a teaching to induce residual stress at that level.
Poyner also states that a portion of the “ball striking surface 24 may be peen treated as well, for example, to remove [alpha]-case.” Poyner, column 5, lines 52-60. However, no discussion of the process used for the ball striking surface is provided.
Poyner also states, without discussion, that laser shock peening and abrasive waterjet peening could be applied. Poyner, column 6, lines 16-39.
Any treatment of golf clubs is subject to scrutiny by the United States Golf Association and other similar associations that regulate the manufacturing of golf clubs to insure fair play. See, Procedure for Measuring the Flexibility of a Golf Clubhead, USGA, Revision 2.0, Mar. 25, 2005. One key factor in golf club manufacturing is compliance with such regulations. Poyner found it relevant for example, to point out the golf clubs made as he suggested “approach the target coefficient of restitution of 0.829 (for a relative velocity of 160 ft/sec), which corresponds to the regulated value established by the United States Golf Association.” Poyner, column 5, lines 35-39. The United States Golf Association USGA and other similar associations regulate the manufacturing of golf clubs to insure fair play. One key factor in golf club manufacturing is compliance with such regulations, including Rule 5a, Appendix II of the Rules of Golf, published by the USGA. Poyner found it relevant for example, to point out the golf clubs made as he suggested “approach the target coefficient of restitution of 0.829 (for a relative velocity of 160 ft/sec), which corresponds to the regulated value established by the United States Golf Association.” Poyner, column 5, lines 35-39. To be recognized as acceptable for competition by the USGA and the Royal and Ancient Golf Club of St. Andrews Scotland, a club is limited in the amount of spring-like effect it can create. Spring-like effect is the ability of a clubface to act as a spring (or trampoline), adding extra oomph to a shot. It is tested by measuring the clubface's coefficient of restitution, or COR. This measurement is done using a standardized testing fixture and involves dropping a pendulum against the club face and measuring the time the pendulum is in contact with the face.
If the COR is too high, the club is deemed to act too much like a spring, and is ruled illegal. All clubs have a COR—even persimmon drivers had COR ratings—but all the fuss stemmed from the introduction of high COR metal drivers. Currently, in territories governed by the USGA, the COR limit is 0.830 for all competitions and all handicap rounds. In territories governed by the R&A, the COR limit will be 0.830 as of Jan. 1, 2008. In the meantime, a “condition of competition” is put into effect by the R&A for highly skilled players limiting COR to 0.830 for those players. Others (i.e., recreational players playing handicap rounds or participating in local events in which that condition of competition is not in effect) must adhere to a COR of 0.860 or less.
Another factor monitored by the USGA is known as Characteristic Time which measures flexibility of the club face. See, Procedure for Measuring the Flexibility of a Golf Clubhead, USGA, Revision 2.0, Mar. 25, 2005. Basically, the regulation requires that the Characteristic Time that the head of a pendulum remains in contact with the club face in a test in which the pendulum arm is dropped from a number of set heights, must be less than 239 microseconds.
Although shot peening has been investigated for the purposes of treating golf clubs to harden the surface, reduce the coefficient of friction, induce residual compressive stress and remove [alpha]-case for at least 12 years, no commercial application of the process is known to the present inventor suggesting that the prior art process has failed to provide significant improvement in golf club manufacturing.
It is desirable to improve golf clubs and golf club manufacturing processes, while remaining within the guidelines set for fair play by the golfing associations. Also, it is desirable to improve golf clubs to enhance the playing experience for golfers.